In recent years, as portable electronic devices such as a smartphone or a digital camera are miniaturized and made highly functional, demands for refinement and density increase are increasing for a flexible printed wiring board used in these electronic devices.
A circuit pattern of a flexible printed wiring board is normally formed by patterning a conductive layer on an insulating substrate by etching using a photofabrication method. Therefore, in order to refine the circuit pattern, it is effective to reduce a thickness of the conductive layer on the insulating substrate. Then, a method of using a copper clad laminate provided with thin copper foil and patterning the copper foil as it is without performing thickening plating to the copper foil is conventionally known. Also, in order to increase a density of the flexible printed wiring board, a method of forming an interlayer connecting path that electrically connects the circuit patterns on both surfaces of the insulating substrate using conductive paste is known. These methods will be described more in detail as follows.
First, as a starting material, the copper clad laminate provided with the thin copper foil on the insulating substrate is prepared. Next, the copper foil is patterned as it is without performing thickening plating processing to the copper foil of the copper clad laminate. Thus, the circuit pattern including wiring, a conformal mask and a land or the like is formed. Next, by laser processing using the conformal mask, a conduction hole that passes through the insulating substrate and has the copper foil exposed at a bottom surface is formed. Then, a screen plate and the circuit pattern are positioned, and the conduction hole is filled with conductive paste by a screen printing method. Thereafter, by thermally curing the conductive paste, the circuit patterns on both surfaces of the insulating substrate are electrically connected.
Additionally, a manufacturing method of a circuit board for a purpose of forming a fine conduction hole is described in Patent Literature 1, and a double-sided printed wiring board having an interlayer connecting path formed of conductive paste filled in a conduction hole is described in Patent Literature 2.